The present invention relates to an angle detection device for detecting a rotational angle of a rotary shaft or the like and to a torque sensor incorporating such an angle detection device.
Japanese Laid-Open Patent Publication No. 05-264292 describes an example of an angle detection device 100 that is known in the prior art. As shown in FIG. 1, the angle detection device 100 includes a resolver 101, which is connected to a rotation shaft, and a controller 102, which calculates a rotational angle θ of the rotation shaft from an output signal of the resolver 101.
The resolver 101 includes an excitation coil WE and detection coils WA and WB for two phases (i.e., phase A and phase B). The excitation coil WE is fixed to the rotation shaft so that the excitation coil WE and the rotation shaft are pivotal relative to the detection coils WA and WB of two phases. The axis of the detection coil WA is perpendicular to the axis of the detection coil WB.
The excitation coil WE has one end that is grounded and another end that is connected to the controller 102 via a differential amplifier 103. The detection coils WA and WB each have one end that is grounded and another end that is connected to the controller 102 via respective differential amplifiers 104 and 105. The controller 102 includes a D/A converter 106, A/D converters 107 and 108, and a central processing unit (CPU) 109. The CPU 109 includes an excitation amplitude signal calculator 110, an output table 111, an amplitude calculator 112, and a rotational angle calculator 113.
In accordance with an excitation synchronizing signal (command) from a timing generator (not shown) of the CPU 109, the excitation amplitude signal calculator 110 generates an excitation amplitude signal DE and sends the excitation amplitude signal DE to the D/A converter 106 via the output table (buffer) 111. The D/A converter 106 converts the excitation amplitude signal DE to an analog signal, or an excitation voltage VE. The excitation voltage VE is amplified by the differential amplifier 103 and applied to the excitation coil WE of the resolver 101. When the excitation coil WE is excited, output signals (detection voltages) VA and VB are respectively induced in the detection coils WA and WB. The detection signal VA is a SIN phase, and the detection signal VB is a COS phase.
The detection signals VA and VB are amplified by the differential amplifier 104 and 105 and sent to the A/D converters 107 and 108, respectively. The A/D converters 107 and 108 convert the detection signals VA and VB to digital signals DA and DB, respectively, and send the digital signals DA and DB to the CPU 109. In accordance with a sampling request signal from the timing generator, the A/D converters 107 and 108 respectively sample and convert the detection signals VA and VB to digital signals DA and DB and send the digital signals DA and DB to the amplitude calculator 112 of the CPU 109.
Based on the digital signals DA and DB, the amplitude calculator 112 calculates a SIN phase amplitude (amplitude of detection signal VA) and a COS phase amplitude (amplitude of detection signal VB). Then, the amplitude calculator 112 sends the SIN phase amplitude and the COS phase amplitude to the rotational angle calculator 113. The rotational angle calculator 113 calculates the rotational angle θ from the SIN phase amplitude and the COS phase amplitude.
The angle detection device 100 is used with another angle detection device to configure a torque sensor that detects steering torque in, for example, an electric power steering apparatus. In other words, two angle detectors are used to detect the rotational angle θ of an input shaft located on a steering wheel side and the rotational angle θ of an output shaft located on the steering gear side. The steering torque applied to the steering wheel is then obtained from the difference between the two rotational angles θ.
However, the angle detection device 100 of the prior art has a shortcoming. In the resolver 101, the excitation current of the excitation coil WE relative to a predetermined excitation voltage VE fluctuates according to conditions such as the ambient temperature or the temperature of the excitation coil WE. This causes fluctuation in the amplitude of the detection signals VA and VB of the resolver 101.
More specifically, when the temperature rises and increases the resistance of the excitation coil WE, the excitation current of the excitation coil WE corresponding to the excitation voltage VE decreases. Thus, the amplitudes of the detection signals VA and VB induced in the secondary side detection coils WA and WB also decrease. This decreases the accuracy of the angle detection device 100 for detecting the rotational angle θ.
The amplitude calculator 112 of the CPU 109 retrieves the detection signals VA and VB via the A/D converters 107 and 108 for a predetermined number of times and calculates the amplitudes of the detection signals VA and VB using least squares. A decrease in the amplitudes of the detection signals VA and VB reduces the resolution of the A/D converters 107 and 108. This further reduces the amplitude calculation accuracy of the amplitude calculator 112 and the rotational angle θ calculation accuracy of the rotational angle calculator 113. Thus, when using the angle detection device 100 as the torque sensor, abnormal noise or vibrations may be produced by an increase in the ambient temperature or the temperature of the excitation coil WE.